An electric power steering apparatus that energizes a steering apparatus of a vehicle by using a rotational torque of a motor as an assist torque, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism (reduction ratio=Gr). In order to accurately generate the assist torque (steering assist torque), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and the motor current becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a pulse width modulation (PWM) control.
A general constitution of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, handle shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3 (reduction ratio=Gr), universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Tr of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on the steering torque Tr detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage command value E obtained by performing compensation and so on with respect to the current command value in a current control section. Furthermore, it is also possible to receive the vehicle speed Vel from a CAN (Controller Area Network) and so on.
The control unit 30 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tr detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 decides a current command value Iref1 that is the target value of the current supplied to the motor 20 based on the steering torque Tr and the vehicle speed Vel and by means of an assist map or the like. The current command value calculating section 31 may include a phase compensating section to improve the stability of the steering system, a center responsibility improving section to secure the stability at a dead band of the assist characteristic and to compensate the static friction, a robust stability compensating section to remove a peak value of the resonance frequency of the resonant system comprising an inertia element and a spring element including the detected torque and to compensate a phase shift of the resonance frequency for obstructing the stability and the responsibility of the control system and so on.
The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A as a current command value Iref2. A current command value Iref3 that is limited the maximum current, is inputted into a subtracting section 32B, and a deviation Iref4 (=Iref3−Im) between the current command value Iref3 and a motor current value Im that is fed back, is calculated. The deviation Iref4 is inputted into a PI-control section 35 serving as the current control section to improve the characteristic of the steering operation. The voltage command value E that characteristic improvement is performed in the PI-control section 35, is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a drive section. The current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. In general, the inverter 37 uses EFTs as switching elements and is comprised of a bridge circuit of FETs.
The current limiting section 33 is not indispensable.
Further, a compensation signal CM from a compensating section 34 is added to the adding section 32A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 to an adding section 344, further adds the result of addition performed at the adding section 344 and a convergence 341 in an adding section 345, and then outputs the result of addition performed at the adding section 345 as the compensation signal CM.
In such the electric power steering apparatus, an operation is suffocated due to the friction of the reduction gears and the pinion and rack mechanism for transmitting the assist force. In spite of the vehicle running state in order to return to the straight running state, there is a case that the handle does not return to the neutral point and therefore the vehicle hardly becomes to the straight running state.
As a prior art for controlling an assist characteristic at an active-return time, the active-return control disclosed in Japanese Patent No. 4872298 (Patent Document 1) is proposed. In the control constitution disclosed in Patent Document 1, a target steering velocity is calculated from the steering angle, the vehicle speed and the steering torque and the compensation current is calculated in accordance with the deviation between the real motor angular velocity/Gr ration ratio and the target steering velocity